... the second suggested solution--using the IO::Insitu module--does use a back-up strategy to ensure that data is not lost if the program abends.

True. But it is still not re-runnable. Which makes it dangerous in the hands of naive users who interrupt a program with CTRL-C, then re-run it.
If they do that, they may suffer permanent data loss and
without being aware of it.

It seems to me that you can get re-runnability with little
extra effort: simply write the temporary file first and
only overwrite the original (via atomic rename) after
the temporary has been successfully written.

Now, of course, blindly re-running the test program resulted
in permanent data loss (an empty fred.tmp file in this example).

Update: Just to clarify, this problem is
broader than the naive user scenario given above
and may bite you anytime a script is automatically
rerun after an interruption -- a script that is run
automatically at boot time, for example.